In the aircraft industry the removal of paint from metal or composite substrates has always been a labor intensive effort. For many years chemical stripping has been the prevalent procedure. It is a time consuming and expensive procedure. In addition, one of the serious drawbacks associated with chemical stripping is the hazard to personnel and the environment due, for example, to the generation of toxic waste. A less hazardous, yet economical alternative has been sought in the aircraft industry.
Some alternatives to chemical stripping for paint removal have been proposed, known as mechanical paint removal procedures, such as laser, flash lamp, high pressure CO.sub.2 and high pressure water knives. These procedures are not entirely acceptable however because of the substrate damage which frequently occurs.
One further alternative which is gaining wide acceptance is low pressure abrasive blast cleaning utilizing plastic abrasives. A brief discussion of this alternative can be found in the SAE Technical Paper Series No. 850713 given at the 21st annual Airline Plating and Metal Finishing Forum in Atlanta Ga. on Feb. 18-21, 1985 by Mr. Joseph Gardner and entitled Abrasive Blast Cleaning-A Viable Alternative To Chemical Stripping Operations.
As with any procedure, one of the concerns with the use of any form of abrasive material is substrate damage. It is essential that aircraft surfaces being stripped of a coating such as paint, and in particular surfaces of parts which are subject to load, such as fatigue load, not be damaged in any way. For example, the introduction of a crack, even a microscopic crack, in a part subject to fatigue loading can result in rapid crack propagation under load leading to failure of the part.
For this reason, a plastic blasting media (PBM) has been proposed. (PBM) is a relatively soft material with sharp angular edges that are provided with excellent cutting qualities. The inherent softness of the material, on the other hand, avoids damage to the substrate.
A detailed description of a typical system shown in FIG. 1 is presented hereinafter.
The use of the system shown in FIG. 1 has proven to be advantageous from the standpoint of man hours saved. It has also proved effective because of the use of (PBM) in avoiding substrate damage.
The (PBM) in such a system is intended to be recycled, and the system shown in FIG. 1 provides for such recycling by separating the (PBM) from the debris generated during the blasting process. In has been found, however, that the present separation process is not entirely satisfactory. A residue of contamination, continues to exist, which can, with cumulative cycles of use, cause the substrate damage noted above.
An object of the present invention, therefore, is to provide a method and apparatus which can be used, in one application, in conjunction with the system shown in FIG. 1 for effectively separating the (PBM) from all debris resulting from a stripping process comprising one or more cycles.